1. Field of the Invention
The present invention relates to a liquid crystal display (hereinafter, referred to as an xe2x80x9cLCDxe2x80x9d) device which is suitable to uses requiring a large viewing angle and to realize large-screen display, and a plasma-addressed LCD device using such an LCD device.
2. Description of the Related Art
(1) ASM Mode
Conventional LCD devices, as represented by TN (twisted nematic) mode LCD devices, cause liquid crystal molecules in the vicinity of two substrates to be aligned in one direction. Accordingly, as shown in FIG. 17, the liquid crystal molecules are tilted in one uniform direction when a voltage is supplied. As a result, the apparent phase difference of the liquid crystal molecules is significantly different when the LCD device is seen in a direction of arrow A and when being seen in a direction of arrow B. Therefore, the transmittance is different when the LCD device is seen in the direction of arrow A and when being seen in the direction of arrow B. Thus, the conventional LCD devices have anisotropy in the viewing angle characteristic.
In order to solve the problem, Japanese Laid-Open Publication No. 6-301015 discloses an ASM (axially symmetrically aligned microcell) mode for realizing a large viewing angle, by which liquid crystal molecules are aligned in an axially asymmetrical manner in each pixel. The ASM mode uses a p-type liquid crystal material. In a vertical plane, the liquid crystal molecules in the vicinity of the substrates are aligned substantially parallel to the substrates. In a horizontal plane, the liquid crystal molecules are aligned in an axially symmetrical manner in each of a top portion (FIG. 18A), an intermediate portion (FIG. 18B), and a bottom portion (FIG. 18C). By applying avoltage to the liquid crystal layer in this state, the liquid crystal molecules are tilted vertically to the substrates, and thus black display is provided.
Japanese Laid-Open Publication No. 7-120728, for example, discloses a method for producing an LCD device of the ASM mode. According to this method, a structure having a wall in a lattice pattern is formed on one of the two substrates, and the liquid crystal molecules are aligned in an axially asymmetric manner by interaction of the wall with the liquid crystal molecules.
These LCD devices of the ASM mode require a technology for polymerizing a photo-curable monomer in order to stably maintain the liquid crystal molecules.
Liquid crystal molecules of an n-type liquid crystal material can also be aligned in an axially asymmetrical manner. For example, Japanese Laid-Open Publication No. 10-133206 discloses realizing the ASM mode by using an n-type liquid crystal material together with a vertical alignment layer.
An LCD device of the ASM mode is produced in the following manner. A substrate is formed so as to have convex or concave portions in a lattice pattern on a surface thereof, and a vertical alignment layer is provided thereon. The substrate is assembled with another substrate having a vertical alignment layer provided thereon. A mixture of an n-type liquid crystal material and a photo-curable monomer is injected into a space between the two substrates to produce a cell. A voltage is applied to the n-type liquid crystal material to align liquid crystal molecules of the n-type liquid crystal material in an axially symmetrical manner. Then, the cell is irradiated with ultraviolet light to cure the photo-curable monomer. Thus, the axial symmetrical alignment is secured. Due to the secured axial symmetrical alignment, the direction in which the liquid crystal molecules are tilted when a voltage is applied is determined, which raises the response speed. As can be appreciated, the AMS mode using the n-type liquid crystal material also indispensably requires the technology for polymerizing a photo-curable monomer in order to stably maintain the liquid crystal molecules.
Japanese Laid-Open Publication No. 7-318940 discloses another method for producing an LCD device of an AMS mode. According to this method, a structure having a wall in a lattice pattern is provided on a substrate having electrodes, and a vertical alignment layer or a horizontal alignment layer is provided thereon. A stable axially symmetrical alignment is obtained by the function of the structure. Japanese Laid-Open Publication No. 7-318940 also discloses a hybrid-type axially symmetrical alignment realized by using a vertical alignment layer together with a horizontal alignment layer.
However, the method disclosed in this publication has a problem that since a transparent electrode is below the structure having a wall, a voltage is unlikely to be applied to a portion of the liquid crystal molecules which is above the wall of the structure. Thus, the contrast is lowered.
In the above-mentioned AMS mode LCD devices, a phase plate having a negative anisotropy is provided between the cell and a polarizer in order to further improve the viewing angle characteristic in a direction of 45 degrees relative to the polarization axis of the polarizer.
(2) PALC
For example, Japanese Laid-Open Publication No. 1-217396 discloses a plasma-addressed liquid crystal (PALC) display device. The PALC display device includes a plasma substrate. The plasma substrate is defined by a substrate and a thin dielectric layer. A space between the substrate and the thin dielectric layer accommodates ribs for dividing the space into a plurality of plasma chambers. The plasma chambers each accommodate an anode electrode and a cathode electrode. A change in the plasma state of a noble gas contained in the plasma chamber provides a switching function. The PALC display device further includes a counter substrate having a counter electrode. The counter substrate and the dielectric layer interpose a liquid crystal layer. A voltage is applied to the liquid crystal layer to perform display.
The PALC technology is favorably expected to be applied to large-screen display due to a simple structure realized by the technology, but has a problem in the viewing angle characteristic since the liquid crystal material is used as an optical switch for display.
In order to solve the problem, Japanese Laid-Open Publication Nos. 9-19738 and 10-186331 disclose applying an AMS mode to a PALC display device.
Hereinafter, the problems of the above-described conventional LCD devices will be described.
(1) Contrast
The hybrid ASM mode described in Japanese Laid-Open Publication No. 7-318940 includes a structure having a wall in a lattice pattern on an electrode. Accordingly, the alignment of a portion of the liquid crystal molecules which is above the wall is not sufficient in response to the voltage application. Thus, the transmittance of the corresponding area is not lowered. As a result, a sufficiently high contrast is not provided. As a solution to the problem, a structure having a black wall in a lattice pattern (black matrix) is provided to shield the area from light. In an LCD device of an axially symmetrical alignment mode, the alignment of the liquid crystal molecules is determined by the alignment force provided by the wall of the structure. Since an alignment force cannot regulate the alignment of the liquid crystal molecules in a large pixel area of 200 xcexcm or greater, such a large pixel area is divided into smaller regions and the structure having a black wall in a lattice pattern is formed in correspondence with the smaller regions. As a result, the numerical aperture is reduced.
(2) Response Speed
In the conventional ASM mode LCD devices, two substrates are both covered with a horizontal alignment layer or a vertical alignment layer. Since the alignment is stable when no voltage is applied, the response speed of the liquid crystal molecules when a voltage is applied is significantly lower than the response speed when the voltage is dropped. Moreover, a portion of the liquid crystal layer on the wall is less likely to be supplied with a voltage than the rest of the liquid crystal layer. Accordingly, when a voltage is applied to the entire liquid crystal layer, the portion on the wall has a lower response speed than the rest of the liquid crystal layer.
(3) Application of the ASM Mode to PALC Display Devices
There are the following problems in applying the ASM mode to the PALC display devices.
(i) An n-type liquid crystal material used in an ASM mode operates in a normally black mode; i.e., is put into a white state (light transmission state) by voltage application. In the case of a PALC display device, a voltage is applied to a liquid crystal layer by striped transparent electrodes as shown in FIG. 19. The applied voltage is as high as several tens of volts. Due to such a high level of voltage, a capacitance coupling occurs between ON-state electrodes and OFF-state electrodes (i.e., a transverse electric field) and thus puts the edge portions of the OFF-state electrodes into an ON state as indicated with reference numeral 19a. Pixel areas corresponding to such portions allow light transmission. Such a phenomenon is conspicuous in highly precise liquid crystal panels such as, for example, high definition panels, and influences the normally black mode display as a color shift.
(ii) In the case of PALC display devices, the thickness of the liquid crystal layer (cell thickness) is set to be larger than that of the display devices including TFTs or the like, in order to guarantee application of a sufficient level of voltage to the liquid crystal layer. Accordingly, the response speed is lower whether a p-type or n-type liquid crystal material is used.
The present inventors performed active researches in order to provide an LCD device which realizes stable axially symmetrical alignment without using a photo-curable monomer to provide a large viewing angle, which prevents reduction in the numerical aperture to provide high contrast display even though pixel areas are divided, and which has a high response speed. As a result, they have found that the above-described LCD device is realized by providing a convex-concave structure for regulating the alignment of liquid crystal molecules on a electrode in a hybrid-type ASM mode having a vertical alignment layer on one substrate and a horizontal alignment layer on the other substrate. The present inventors also studied on a device without a horizontal alignment layer in an attempt to reduce the number of elements of the device. The present inventors also found that some of the elements which are usually included in an LCD device are usable as the convex-concave structure for regulating the alignment.
According to one aspect of the invention, a liquid crystal display device includes a first substrate having a vertical alignment layer provided thereon; a second substrate having a horizontal alignment layer provided thereon; a liquid crystal layer including liquid crystal molecules interposed between the first substrate and the second substrate; and a concave-convex structure for regulating an alignment direction of the liquid crystal molecules and a transparent conductive layer provided on the concave-convex structure, the concave-convex structure being provided on one of the first substrate and the second substrate with one of the vertical alignment layer and the horizontal alignment layer interposed therebetween. The liquid crystal molecules in the vicinity of the horizontal alignment layer are aligned in an axially symmetrical manner when no voltage is provided, and the liquid crystal molecules in the liquid crystal layer are aligned substantially vertically relative to the first substrate and the second substrate when a voltage is applied.
In one embodiment of the invention, the convex-concave structure is provided on the second substrate with the horizontal alignment layer interposed therebetween.
According to another aspect of the invention, a liquid crystal display device includes a first substrate; a second substrate having a vertical alignment layer provided thereon; a liquid crystal layer including liquid crystal molecules interposed between the first substrate and the second substrate; a concave-convex structure for regulating an alignment direction of the liquid crystal molecules and a transparent conductive layer provided on the concave-convex structure, the concave-convex structure being provided on the first substrate, and the transparent conductive layer being in contact with the liquid crystal layer. The liquid crystal molecules in the vicinity of the transparent conductive layer are aligned in an axially symmetrical-manner when no voltage is provided, and the liquid crystal molecules in the liquid crystal layer are aligned substantially vertically relative to the first substrate and the second substrate when a voltage is applied.
In one embodiment of the invention, the liquid crystal layer is a p-type liquid crystal material.
In one embodiment of the invention, the convex-concave structure has convex portions provided in a lattice pattern.
In one embodiment of the invention, the convex-concave structure has convex portions provided in a checkered pattern.
In one embodiment of the invention, the convex-concave structure has concave portions provided in a lattice pattern.
In one embodiment of the invention, the convex-concave structure has concave portions provided in a checkered pattern.
In one embodiment-of the invention, the liquid crystal display device further includes a phase plate having a negative refractive index anisotropy outside at least one of the first substrate and the second substrate.
In one embodiment of the invention, the liquid crystal display device further includes a plurality of switching elements, and a plurality of source lines and a plurality of gate lines for driving the plurality of switching elements. The plurality of switching elements, the plurality of source lines and the plurality of gate lines are provided on one of the first substrate and the second substrate. The plurality of source lines and the plurality of gate lines act as a convex-concave structure having convex portions.
In one embodiment of the invention, the liquid crystal display device further includes a layer having a plurality of portions of different colors and a black matrix portion having a greater height than a height of the plurality of portions of different colors, wherein the black matrix portions act as a convex-concave structure having convex portions.
In one embodiment of the invention, the black matrix portions are formed of a resin.
According to still another aspect of the invention, a plasma-addressed liquid crystal display device is provided. The first substrate of the above-described liquid crystal display device is formed of a plasma section. The plasma section includes a third substrate; a dielectric layer interposed between the second substrate and the third substrate; a plurality of ribs for dividing a space between the dielectric layer and the third substrate into a plurality of plasma chamber containing a noble gas; and an anode electrode and a cathode electrode accommodated in each of the plasma chambers. Voltage switching is performed by changing the plasma state of the noble gas.
According to still another aspect of the invention, a plasma-addressed liquid crystal display device is provided. The second substrate of the liquid crystal display device is formed of a plasma section. The plasma section includes a third substrate; a dielectric layer interposed between the first substrate and the third substrate; a plurality of ribs for dividing a space between the dielectric layer and the third substrate into a plurality of plasma chamber containing a noble gas; and an anode electrode and a cathode electrode accommodated in each of the plasma chambers. Voltage switching is performed by changing the plasma state of the noble gas.
According to still another aspect of the invention, a plasma-addressed liquid crystal display device is provided. The second substrate of the liquid crystal display device is formed of a plasma section. The plasma section includes a third substrate; a dielectric layer interposed between the first substrate and the third substrate; a plurality of ribs for dividing a space between the dielectric layer and the third substrate into a plurality of plasma chamber containing a noble gas; and an anode electrode and a cathode electrode accommodated in each of the plasma chambers. Voltage switching is performed by changing the plasma state of the noble gas.
According to still another aspect of the invention, a plasma-addressed liquid crystal display device includes a first substrate; a second substrate; a dielectric layer provided between the first substrate and the second substrate; a liquid crystal layer including liquid crystal molecules provided between the second substrate and the dielectric layer; a plurality of ribs for dividing a space between the dielectric layer and the first substrate into a plurality of plasma chambers containing a noble gas; an anode electrode and a cathode electrode accommodated in each of the plurality of plasma chambers; a vertical alignment layer provided on one of the second substrate and the dielectric layer so as to be in contact with the liquid crystal layer; a horizontal alignment layer provided on the other of the second substrate and the dielectric layer so as to be in contact with the liquid crystal layer; and a convex-concave structure for regulating an alignment direction of the liquid crystal molecules, the convex-concave structure being provided on one of the second substrate and the dielectric layer with one of the vertical alignment layer and the horizontal alignment layer interposed therebetween. The liquid crystal molecules in the vicinity of the horizontal alignment layer are aligned in an axially symmetrical manner when no voltage is applied, and the liquid crystal molecules in the liquid crystal layer are aligned substantially vertically relative to the second substrate and the dielectric layer when a voltage is applied. Voltage switching is performed by changing the plasma state of the noble gas.
According to still another aspect of the invention, a plasma-addressed liquid crystal display device includes a first substrate; a second substrate; a dielectric layer provided between the first substrate and the second substrate; a liquid crystal layer including liquid crystal molecules provided between the second substrate and the dielectric layer; a plurality of ribs for dividing a space between the dielectric layer and the first substrate into a plurality of plasma chambers; an anode electrode and a cathode electrode accommodated in each of the plurality of plasma chambers containing noble gas; a convex-concave structure for regulating an alignment direction of the liquid crystal molecules and a transparent conductive layer provided on the convex-concave structure, the convex-concave structure being provided on the second substrate, and the transparent conductive layer being in contact with the liquid crystal layer; and a vertical alignment layer provided on the dielectric layer. The liquid crystal molecules in the vicinity of the transparent conductive layer are aligned in an axially symmetrical manner when no voltage is applied, and the liquid crystal molecules in the liquid crystal layer are aligned substantially vertically relative to the second substrate and the dielectric layer when avoltage is applied. Voltage switching is performed by changing the plasma state of a noble gas.
According to the present invention, the liquid crystal molecules in the vicinity of the inclined convex-concave structure are aligned along the inclination. Therefore, stable axially symmetrical alignment is obtained simply by injecting the liquid crystal material, without using a photo-curable monomer. Due to provision of a vertical alignment layer on one substrate and a horizontal alignment layer on the other substrate, the liquid crystal molecules are prepared to be tilted when no voltage is applied. Therefore, the response speed is higher than the conventional devices having a vertical alignment layer or a horizontal alignment layer on both of the substrates. Since the convex-concave structure is provided between a transparent conductive layer and the substrate, a portion of the liquid crystal layer on a convex portion of the convex-concave structure is supplied with a sufficient voltage to be tilted, thus avoiding contrast reduction. Even in the case of a device having a large pixel area, the pixel area can be divided into a plurality of smaller regions without reducing the numerical aperture when the convex-concave structure is light-transmissive.
In an embodiment where the convex-concave structure is provided on the substrate having a horizontal alignment layer, the alignment of the liquid crystal molecules are stabilized.
In an embodiment where a p-type liquid crystal material is used, the liquid crystal molecules in the vicinity of the horizontal alignment layer are aligned in an axially symmetrical manner when no voltage is applied. When a voltage is applied, the liquid crystal molecules in the liquid crystal layer are aligned substantially vertically to the substrates. By providing polarizers so that the polarization axes thereof are perpendicular to each other, the device operates in a normally white mode in which the transmittance is maximized when no voltage is applied. Thus, bright display is obtained with less power consumption.
In an embodiment where the convex or concave portions of the convex-concave structure are arranged in a lattice pattern or a checkered pattern, the size of regions in which the liquid crystal molecules are aligned in an axially symmetrical manner corresponds to the pixel area.
In an embodiment where a phase plate having a negative refractive index anisotropy is provided between the cell and the polarizer, the viewing angle characteristic in the direction of 45 degrees with respect to the polarization axis of the polarizer is further improved.
A transparent conductive layer (ITO or the like) has a surface free energy sufficiently large to align the liquid crystal molecules in the vicinity thereof in a horizontal direction. Therefore, the transparent conductive layer can make a separate horizontal alignment layer unnecessary.
In a TFT LCD device, the gate lines and source lines are provided so as to intersect each other and can form a stepped concave-convex structure shown in FIG. 16 described below. Thus, a separate convex-concave structure for regulating the alignment does not need to be formed with expensive photolithography or the like. Nor does the convex-concave structure formed of the gate lines and source lines reduce the numerical aperture. The metal materials, transparent conductive materials, and glass used in the TFT LCD devices have a surface free energy sufficiently large to align the liquid crystal molecules in the vicinity thereof in a horizontal direction. Therefore, a separate horizontal alignment layer can be eliminated.
In an embodiment where a color filter substrate having a black matrix higher than a color layer is included as shown in FIG. 15 described below, the black matrix can be used as a convex-concave structure. Such a black matrix does not reduce the numerical aperture, and can be produced by use of a resist material.
In a PALC display device, even when the pixel area is divided, the numerical aperture is not reduced as long as the convex-concave structure is formed of a light-transmissive material. The use of the hybrid-type ASM mode provides a larger viewing angle and a higher response speed. Since the PALC display device is usually operated at a relatively high voltage, provision of the convex-concave structure on the transparent conductive layer does not reduce the contrast. Since it is difficult to provide a convex-concave structure on a dielectric layer of the plasma substrate for production reasons, the convex-concave structure is preferably provided on the substrate facing the dielectric layer with the liquid crystal layer interposed therebetween.
In the case of a PALC display device also, the transparent conductive layer can be used as a horizontal alignment layer, and the black matrix can be used as convex portions of the convex-concave structure. Thus, the number of elements can be decreased to reduce the cost.
Thus, the invention described herein makes possible the advantages of providing a liquid crystal display device, and a PALC display device using such an LCD device, which realizes stable axially symmetrical alignment without using a photo-curable monomer to provide a large viewing angle, which prevents reduction in the numerical aperture to provide high contrast display even though pixel areas are divided, and which has a high response speed, at a low cost.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.